1. Field of the Invention
This invention relates to a ventilation system for an engine, and more particularly to a blow-by gas ventilation system suitable for a marine drive.
2. Description of Related Art
An internal combustion engine typically includes a cylinder body defining at least one cylinder bore. A piston reciprocates in the cylinder bore. A cylinder head is affixed to one end of the cylinder body and defines a combustion chamber with the cylinder bore and the piston. A crankcase member is affixed to the other end of the cylinder body and defines a crankcase chamber with the cylinder body. A crankshaft is journaled within the crankcase chamber and is connected to the piston for rotation with the reciprocal movement of the piston. The piston is provided with at least one piston ring to isolate the combustion chamber from the crankcase chamber.
Ideally, combustion gases and unburnt air/fuel charges in the combustion chamber never blow by the piston rings into the crankcase chamber. In practice, however, some combustion gases and unburnt charges (hereinafter collectively referred to as "blow-by gases") are blown by the piston ring into the crankcase chamber during combustion. A two-stroke engine allows the blow-by gases to return to the combustion chamber through scavenge passages for burning. In a four-stroke engine, however, the blow-by gases accumulate in the crankcase chamber or in a lubricant reservoir, which is usually coupled with the crankcase chamber. Accordingly, the pressure in the crankcase chamber and the lubricant reservoir inevitably increases. This high pressure decreases engine output. Additionally, deterioration of the lubricant, including a viscosity decrease, and oil leakage from an oil inlet, oil gauge and the like, due to the increased pressure can occur.
In order to resolve these problems, conventional engines are typically provided with ventilation systems. A compulsory circulation type ventilation system is known as one of such systems. This type of system employs an air circulation unit that introduces ambient air via an air induction device and compels it into flowing through the crankcase and lubricant reservoir to sweep away the blow-by gases therein to the air induction device. Although this system is quite effective, a large space is necessary for its application.
Outboard motors today commonly employ four-stroke engines. The engines of the outboard motors are usually surrounded by protective cowling assemblies and hence such large size ventilation systems are not suitable for the outboard motors.
Ventilation systems in simple and compact constructions are, therefore, employed with the outboard motors. The systems have no compulsory air circulation units but simply have blow-by gas conduits that couple the crankcases to the air induction devices, specifically, to plenum chambers. The blow-by gases are led to oil separators or breathers that are provided to remove an oil component. After the oil separators remove the oil component contained in the gases, the blow-by gases are distributed to the combustion chambers through the air induction devices to be burnt in the combustion chambers.
Many engines have a multiple combustion chambers because of the requirement for larger outputs. The air induction devices for such engines have plurality of air delivery conduits that communicate with the respective combustion chambers. In the outboard motors, the delivery conduits extend generally vertically and are spaced apart from each other. If, for example, the blow-by gas conduits are connected atop of the plenum chambers, the blow-by gases are likely to flow into the upper delivery conduits rather than the lower conduits. Due to the imbalance flow of the blow-by gases, the air/fuel ratio of the air/fuel charges delivered to the respective combustion chambers are different from each other. As a result, the combustion conditions for the respective combustion chambers are dissimilar and hence, engine components such as pistons, connecting rods and bearings should have different endurance in each cylinder. This difference in the air/fuel ratio delivered to the cylinder can also produce an imbalance between the power outputs from the cylinders which degrades engine performance. A need therefore exists for an improved blow-by gas ventilation system that generally distributes the blow-by gas evenly between the cylinders.